Wastewater treatment

ABSTRACT

Improved clarity in treated wastewater is achieved by adding a flocculating agent following screening by fine mesh screening means, preferably a centrifugal screen separator. The effectiveness of the separator is improved, in turn, by preceding it in the direction of liquid flow by a corrugated plate interceptor to remove greases and oils. Preferably, fine mesh screening is carried out by two separate screening devices, one being a high liquid volume centrifugal screen strainer, and the other being a mechanically self-cleaned low liquid volume device. Wastewater is delivered to a thickener, preferably a corrugated plate separator, the thickened product being delivered to the low liquid volume device, and the supernatant being delivered to the centrifugal screen strainer. The corrugated plate interceptor not only acts as a thickener, but also eliminates grease and oil, which would interfere with the operation of the centrifugal screen strainer. 
     Organic sludge produced in the system is recycled through a carbonization system comprising a loop dryer and a molten salt column to produce a high grade activated carbon adsorbent. 
     The throughput of an electrolytic flotation apparatus is enhanced by the incorporation of a corrugated plate interceptor in its outlet, in which reverse flow of particulate matter takes place.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application incorporates by reference the entire disclosure of theU.S. Pat. application of James F. Albus and Scott A. Santora, Ser. No.6,291, filed Jan. 25, 1979 and the entire disclosure of the U.S. Pat.application of Scott A. Santora, Ser. No. 962,996, filed Nov. 22, 1978.

SUMMARY OF THE INVENTION

This invention relates to wastewater treatment, and particularly to acombined physical-chemical wastewater treatment process which is notdependent on biological action for its operation. The general objectiveof the invention is to receive untreated municipal or industrial sewage,and to convert the sewage into a clear and innocuous effluent, whileaccommodating wide variations in BOD and COD loading of the influent andin the temperature and toxicity of the influent, with minimum space andlabor requirements.

In the past, a variety of systems and processes have been proposed forthe physical-chemical treatment of wastewater. Some of these utilizeflocculation for the purpose of separating particulate matter out of theliquid. Flocculation is accomplished by the addition of a flocculatingagent, which usually consists of a combination of coagulants andpolyelectrolytes which function to cause extremely fine particlessuspended in the wastewater to agglomerate, and either float to thesurface or sink to the bottom of the liquid depending on the density ofthe floc.

In general, in processes of this type, the flocculation agent has beenadded to the raw wastewater, or to wastewater from which only very largesolids have been removed by means of bar screens or similar devices.Flocculation agents have also been added to wastewater from which somesolids have previously been allowed to settle out in settling basins. Ineither case, however, the wastewater to which the flocculation agent isadded has a relatively high solids content, and the action of theflocculating agent is unable to achieve good clarity in the effluent. Inaddition, where settling basins are used, land requirements are high.

In accordance with the invention, substantially all of the solid organicsludge in the wastewater is mechanically separated out by passing thewastewater through fine particle screening means. Only thereafter is aflocculating agent added to the wastewater. In the substantial absenceof organic solids, the flocculating agent is able to remove extremelyfine particles of phosphates, oil drops and other substances. Theprocess is preferably carried out by removing substantially all of theorganic solids from the wastewater by first removing coarse particleswith a bar screen, then passing the wastewater through a rotary strainerto effect a further removal of solids, then settling out fine solidsutilizing a corrugated plate interceptor, and finally by passing thewastewater through a vertical centrifugal screen concentrator.Desirably, the centrifugal screen concentrator is designed to removesubstantially all particles above about 100 microns in size. Preferably,the centrifugal screen concentrator also removes a large percentage ofparticles below 100 microns in size.

The flocculating agent is introduced into the liquid passing through theoutlet of the centrifugal screen device. After the addition of theflocculating agent, the wastewater is preferably delivered to anelectrolytic flotation unit, in which extremely fine gas bubbles aregenerated electrolytically. These gas bubbles carry the floc formed bythe action of the flocculating agent to the surface of the liquid whereit is skimmed off. A corrugated plate interceptor is preferably providedat the outlet end of the flotation unit in order to prevent particlesfrom being carried out with the effluent. The combination of the finebubbles produced by electrolytic action and the corrugated plateinterceptor allows the flotation apparatus to operate at an unexpectedlyhigh liquid flow rate, while producing a high quality product.

Since most of the organic solids are removed before addition of theflocculating agent to the wastewater, it is possible to utilizeflotation for the removal of the remaining extremely fine particles.

The use of activated carbon adsorption for the treatment of wastewateris helpful, particularly where the wastewater contains trace amounts ofcolor, taste and odor-producing compounds and other organiccontaminants. Activated carbon is particularly useful, because itremoves nonbiodegradable organic substances and many toxic inorganicmaterials as well. However, it is subject to a number of disadvantages,in particular high equipment costs, and the high costs of the activatedcarbon itself.

Wastewater purification systems have been suggested in which activatedcarbon is produced directly from sludge which is settled out of thewastewater. The activated carbon is produced by pyrolysis in a furnace,is fed into the wastewater which flows out of a sludge settling basin,is settled out of the wastewater in a second settling basin, and isrecycled into the incoming wastewater.

While the manufacture of activated carbon directly from sewage sludgehas certain economic advantages over conventional activated carbon wastetreatment systems, it has the disadvantage that heavy metals areaccumulated in the system, which eventually find their way, in the formof heavy metal salts, into the effluent.

In accordance with the present invention, organic sludge is mechanicallyseparated from the wastewater, and at least part of the sludge isintroduced into a lower portion of a molten salt carbonizer. Thecarbonizer is a molten salt column in which pyrolysis of the sludgetakes place to form an activated carbon which floats to the top of themolten salt, and is there recovered for use. The molten salt carbonizercontinuously removes heavy metal salts from the system by causing themto dissolve and/or sink to the bottom of the column, where they areperiodically removed. The activated carbon which is produced in thissystem is relatively free of heavy metal salts, and, as it isrecirculated in the system through the molten salt carbonizer, any heavymetal salts which are picked up by the activated carbon are againremoved and caused to dissolve and/or sink to the bottom of the column.

The system in accordance with the invention preferably utilizes a finescreen device, such as a vertical centrifugal screen concentrator, forthe removal of very fine particles just prior to the introduction of theflocculating agent. The centrifugal screen concentrator, and similarfine screen devices are highly effective in removing particulate matterfrom wastewater. The centrifugal screen device, in particular, iscapable of accommodating high liquid flow rates while separating outvery fine particles. The presence of oils and greases in the wastewaterinterferes with the effective operation of centrifugal screenconcentrators and other fine screen devices of the type lackingprovisions for mechanical self-cleaning. In accordance with theinvention, oils and greases are effectively separated from thewastewater before it reaches the fine screen device preferably by meansof a corrugated plate interceptor, a separation device which causes oilsand greases to float to the surface of the wastewater, where they areskimmed off for later removal. These oils and greases are preferablydelivered along with settled sludge to a mechanically self-cleaned finescreen device, the output of which is recombined with the liquid outputof the centrifugal screen device.

The centrifugal screen concentrator is a relatively high volume deviceso far as liquid flow rates are concerned. However devices of this typegenerally have the limitation that their solids output is accompanied byconsiderable quantities of water. In a typical centrifugal screenconcentrator, for example, the water accompanying the solids output isbetween five and ten percent of the effluent volume.

Mechanically self-cleaned fine screen devices, on the other hand, aregenerally capable of accommodating only relatively low liquid flowrates, but are adapted to deliver comparatively dry solids. Inaccordance with the invention, a mechanically self-cleaned fine screendevice receives the solids output from the centrifugal screen strainer,and the effluents of both devices are combined for maximum recovery ofwater.

In the preferred form of the invention the solids output of thecentrifugal screen strainer, instead of being delivered directly to themechanically self-cleaned device, is directed to the separation devicecontaining the corrugated plate interceptor. The separation deviceinherently acts as a thickener for the solids so that thickened solidsare delivered to the self-cleaning device, improving its efficiency ofoperation. The separation device thus serves a dual purpose: iteliminates oils and greases from the influent to the centrifugal screenstrainer, and thus improves its operation; and at the same time itthickens the solids output of the centrifugal screen strainer for moreefficient operation of the mechanically self-cleaned device.

The flocculating agent, which is introduced into the effluent of thefine screen devices, is recovered by skimming the surface of theelectrolytic flotation unit, and dewatering the skimmed solids by meansof a tube filter press. The coagulants, which are typically ferricchloride or aluminum sulfate, are converted to hydroxides in theflotation unit so that the filter cake contains high percentages ofmetal hydroxides such as ferric hydroxide or aluminum hydroxide. Thefilter cake is flash dried at high temperatures to destroy any bacteriapresent, and is then pelletized. The pelletized materials, which containhigh concentrations of metals, can be used as ores for steel or aluminumproduction, or alternatively can be refined for reuse as coagulants inthe system.

In the system, activated carbon, which is the adsorbent for organiccontaminants, is preferably introduced at the earliest possible stagefor maximum contact with the wastewater. Desirably, it is introducedinto the effluent of the rotary strainer, and remains in contact withthe liquid until it either floats to the surface or sinks to the bottomof the thickener. The carbon is eventually removed from the systemthrough the mechanically self-cleaned fine screen device for disposal orregeneration. The retention time of the carbon is controllable, becausethe delivery of materials from the surface and from the bottom of thethickener to the mechanically self-cleaned device can be controlled bythe intermittent operation of pumps.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a wastewater treatment system inaccordance with the invention; and

FIG. 2 is a schematic diagram of the molten salt carbonization apparatusused for the generation of activated carbon for use in the treatmentsystem of FIG. 1.

DETAILED DESCRIPTION

Referring to FIG. 1, untreated sewage is first passed through a barscreen 10, which is preferably a screening device of the type describedin U.S. Pat. No. 3,591,006, dated July 6, 1971, the disclosure of whichis incorporated here by reference. This bar screen is provided with anautomatic reciprocating rake which moves over the screen and carriesaccumulated solids up the face of the screen and onto a conveyor. Thebar screen removes debris and very coarse solid material (e.g. allmaterial having a minimum dimension of three inches), delivering them tosludge path 12. The liquid which passes through the screen is deliveredto liquid path 14.

A pump 16, which is preferably a liquid pump of the impellerless type,delivers the screened liquid from path 14, through path 18 to the inlet20 of a rotary strainer generally indicated at 22.

The rotary strainer comprises a rotatable cylindrical screen 24, whichis arranged by means of suitable baffles so that liquid entering headbox 26 through inlet 20 is required to pass into the interior of screen24 and thence outwardly through the screen in order to reach tail box28. Solids intercepted by screen 24 are carried, by rotation of thescreen, to the location of doctor blade 30, which scrapes the solids offthe exterior of screen 24. The solids are thus removed from the screen,and are delivered by means of a conveyor to path 32. The rotary straineris preferably of the type described in Welles, Jr. U.S. Pat. No.3,876,548, dated Apr. 8, 1975, the disclosure of which is incorporatedby reference. The screen is preferably designed to pass solids less than500 microns in diameter and to retain larger solids.

Activated carbon is introduced into tail box 28 through inlet 34 fromactivated carbon path 36. Liquid from tail box 28, along with activatedcarbon, is delivered through tail box outlet 38 and path 40 to inlet 42of a separation vessel 44.

Separation vessel 44 contains a corrugated plate interceptor and variousbaffles for controlling the flow of liquids, including water, variousgreases and oils, and solid materials including solid wastes, andactivated carbon. The principal element within vessel 44 is thecorrugated plate interceptor 46, which is a multiple corrugated platedevice of the type described in Cornelissen U.S. Pat. No. 3,346,122,dated Oct. 10, 1967, which is also incorporated by reference.

Stated briefly, the corrugated plate interceptor is in the shape of arectangular prism, and has an upper opening 48 and a lower opening 50,and is otherwise closed. The interceptor contains an array of corrugatedplates, the corrugations running lengthwise between openings 48 and 50,and the plates being spaced from each other to provide flow paths in thedirection of the corrugations. Gutters (not shown) are provided atopenings 48 and 50 to cause oils and solid materials to flow in thedesired directions as they pass outwardly from between the corrugatedplates.

Baffle 52 and baffle 54, acting together with the closed walls ofinterceptor 46 divide the interior of vessel 44 into two parts, andrequire liquid entering through inlet opening 42 to pass over baffle 54in order to enter opening 48 of the separator. A flow distributionbaffle 56 is provided over inlet opening 48 in order to produce an evendistribution of flow at the inlet of the separator. As wastewater flowsinto vessel 44 through opening 42, some settling of solid material takesplace in sludge compartment 58, and sludge in compartment 58 can beremoved through sludge outlet 60 periodically. Much of the liquid whichpasses over baffle 54 flows downwardly into opening 48 of interceptor46, and outwardly through opening 50 into chamber 62. Solids whichaccumulate at the bottom of chamber 62 can be removed periodicallythrough outlet 64. Liquid which flows over weir 66 at the upper end ofchamber 62 is delivered to path 68. Liquid which passes over weir 70 isdelivered to path 72.

The interceptor operates by reason of the fact that, as liquid passesdownwardly between the corrugated plates, a laminar flow condition ismaintained. Under conditions of laminar flow, oils, greases, and solidswhich are lighter than water, tend to rise and collect in the peaks ofthe corrugations, in which they travel upwardly toward inlet opening 48of the interceptor. The gutters at opening 48 are aligned with thesepeaks, and direct the flow of oils, greases and light solids toward weir70 so that these materials are ultimately delivered to path 72.

Heavier solids, on the other hand, tend to accumulate in the troughs ofthe corrugated plates, and travel downwardly therein toward opening 50.The gutters at opening 50 are aligned with the troughs, and cause thesesolid materials which pass downwardly through opening 50 to be directedtoward the bottom of chamber 62.

Separator vessel 44 performs several important functions. First, it isvery effective in causing settling of solid materials. Secondly, it isvery effective in removing oils and greases from the wastewater enteringthrough inlet opening 42. The latter effect is important, because itmarkedly improves the operation of centrifugal screen concentrator 74,which receives liquid from path 68 through pump 76.

The centrifugal screen concentrator comprises an enclosure 78 in whichis located a fine mesh screen 80 of generally cylindrical configuration.Screen 80 is arranged to be rotated about a vertical axis of rotation bymotor 82. A flow distributor 84 receives liquid through a verticalcentral passage 86 from pump 76, and directs the liquid outwardlyagainst the inner surface of the rotating screen 80. Baffles 88, whichare carried by the screen assembly and rotate with it, deflect liquidwhich splashes off the screen back onto it. A generally cylindricalbaffle 90 which is arranged symmetrically about the axis of rotation ofthe screen assembly, divides the interior of housing 78 into an outerchamber 92 and an inner chamber 94. Backwash spray nozzle assembly 96,which is located within outer chamber 92 is arranged to direct a flow ofwater against the outer surface of rotating screen 80. (Provisions mayalso be made to direct a flow of water against the inner surface of thescreen if desired.) The backwash flow causes solid materials whichaccumulate on the inner surface of screen 80 to fall into chamber 94,along with excess liquid. The solids and liquid are delivered throughoutlet 98 to path 100. Liquid which passes through screen 80 into outerchamber 92 is delivered through outlet 102 to path 104.

Carbon particles are returned from strainer 74 to separator vessel 44through path 100 so that they are maintained in contact with thewastewater for an extended period of time.

Rotating screen 80 is preferably a very fine mesh (i.e. at least 165mesh) screen, and its ability to separate solids from the liquidentering the centrifugal screen concentrator is enhanced by the rotationof the screen. The relative velocity of the liquid flowing outwardlyfrom distributor 84 with respect to the rotating screen 80 is notperpendicular to the screen. Hence, the screen is able to removeparticles smaller than its mesh openings. In a typical installation,particles of less than 44 microns in diameter can be effectivelyremoved. Liquid is forced through the cylindrical screen 80 bycentrifugal action, while solid material, along with some liquid fallsinto inner chamber 94 through the opening in the lower part of therotating screen assembly.

Centrifugal screen concentrators of the type described are availablefrom a number of sources including Sweco, Inc., 6033 East BandiniBoulevard, Los Angeles, Calif. 90051. These concentrators are capable ofhandling relatively high liquid flow rates, but the solids output isaccompanied by relatively large amounts of liquid.

The liquid output of concentrator 74 is delivered through path 104 to anelectrolytic flotation cell 106. Quantities of polyelectrolytes andcoagulants are added to the liquid in path 104, as are quantities of abase such as sodium hydroxide. Suitable polyelectrolytes includeanionic, cationic or non-ionic polyelectrolyte resins such as Nalcolyte607, Nalco 7134, Nalcolyte 7120, Nalcolyte 7182, Nalcolyte 7763 orNalcolyte 603, all available from Nalco Chemcial Company, 180 NorthMichigan Avenue, Chicago, Ill. The choice of the best polyelectrolytedepends on the composition of the wastewater being treated in accordancewith conventional wastewater treatment practice. Coagulants such asferric chloride or aluminum sulfate are also added to alter the "zetapotential" of the liquid in order to allow fine suspended material tobecome neutralized so that it will float in the electrolytic flotationcell.

The flotation cell is a vessel having an array of baffles to control theflow of liquids therein and to direct the movement of solids beingseparated from the liquid by flotation. The liquid in path 104 isintroduced into the left side of the cell into compartment 107 which isdefined by the left wall of the cell and vertical baffle 108, whichextends from above the surface of the liquid in the cell to a locationsufficiently spaced from the bottom of the cell to allow room for anelectrode assembly 110. The electrode assembly preferably comprisesthree layers of foraminous screens, the upper layer 112 and the lowerlayer 114 being of electrically conductive material such as stainlesssteel or graphite, and the intermediate layer 116 being of electricallyinsulating material such as polypropylene, and serving to prevent directcontact between the conductive layers. Conductive layers 112 and 116 areconnected respectively to the negative and positive terminals of adirect current power supply 118, which delivers sufficient voltage andcurrent to produce electrolysis of the liquid with the accompanyinggeneration of minute bubbles of oxygen and hydrogen. (Other gases suchas chlorine may be produced as well depending on the wastewater contentsand on the electrode composition.) These bubbles rise in the liquidabove the electrode screens, and cause the agglomerated solids to riseto the surface of the liquid.

The lower end 118 of sloping baffle 120 is positioned so that most ofthe bubbles generated by the electrodes rise into compartment 122 to theright of baffle 120, and a relatively small proportion of the bubblesrise within left-hand compartment 107. Solids which float upwardly incompartment 107 are eventually collected in compartment 122, which islocated between baffle 108 and weir 124, and are discharged fromcompartment 122 through path 126.

A corrugated plate interceptor 28 is provided within the flotation cell,with its inlet opening 130 in communication with the upper portion ofcompartment 122, and with its lower opening 132 in communication with aright-hand chamber 134, which is separated from compartment 122 by meansof baffles 136 and 138, and by the interceptor itself.

Flow of liquid takes place downwardly through the interceptor fromcompartment 122 into chamber 134, and much of the fine solids whichwould otherwise be carried by the flow of liquid into chamber 134 riseinto the peaks of the corrugations in the interceptor, and reverse theirdirection, whereupon they rise to the surface of the liquid incompartment 122. The solids which rise to the surface of compartment 122are removed by a rotating scraper 140, which carries the solids overbaffle 142 into compartment 144, which is located between baffles 108and 120. An outlet port 146 is provided at the bottom of compartment144, and from it, settled solids are delivered into path 148. The liquidpasses upwardly through compartment 134 and over weir 154 into chamber156, from which it is discharged as a clear effluent through path 158.

In the event of a system upset resulting in the presence, in theelectrolytic flotation cell, of heavy solids which do not float to thesurface, some of these solids are carried into interceptor 128. Theseheavier solids within the interceptor are accumulated in the troughs ofthe plates, and settle at the bottom of compartment 134, where they areremoved through outlet port 150 into path 152.

Occasional solids which accumulate at the bottom of electrolyticflotation cell 106 are discharged through outlet ports 176 and 178,respectively above and below the electrode screens, and through path 180back to the inlet of bar screen 10.

A portion of the effluent in path 158 is delivered by means of pump 160to backwash spray nozzle assembly 96 within concentrator 74. Anoxidizer, such as chlorine or ozone is added to the backwash paththrough path 162.

The pH of the effluent in path 158 is desirably controlled automaticallyby a pH control apparatus 164, which measures the pH of the effluent inpath 158, and controls the flow of the base into path 104 by controllingthe operation of valve 166. In this way, compensation is made for theacidity of the coagulant.

Chemical sludge which accumulates in compartment 144 is deliveredthrough path 148 to a tube filter press 168, which is used to dewaterthe chemical sludge to provide a useful product. The filter press alsoperiodically receives chemical sludge from the bottom of chamber 134through path 152 and from compartment 122 through path 169. A suitabletube filter press is available from Aerodyne Development Corp., 29085Sollon Road, Cleveland, Ohio 44139. The tube filter press consists of anouter cylinder, a rubber bladder and a perforated inner cylindercompletely covered with filter cloth. Hydraulic fluid is used tocompress the bladder inwardly in order to compress the sludge againstthe filter cloth covering the inner cylinder.

Filtration is accomplished from outside to inside, and the perforationsin the inner cylinder conduct the filtrate into the center to be carriedaway through path 170 and delivered by pump 172 back to compartment 107of the electrolytic flotation cell. With the rubber bladder dilatedagainst the outer case of the filter press, a charge of sludge entersthe press through path 148. This charge partly fills the volumecontained within the bladder and the ends of the cloth coveredperforated core. At this time, hydraulic fluid enters between the outercase and the bladder, which compresses the bladder inwardly to commencefiltration. Liquid is forced through the cloth and the perforations inthe core, while solids remain between the bladder and the cloth filter,forming a filter cake. The bladder is again dilated by applying a vacuumbehind it, and the inner cylinder is lowered, whereupon a blast of airremoves the filter cake through path 174. This cycle of operation isthen repeated.

Coagulants such as ferric chloride and aluminum sulfate form ferrichydroxide or aluminum hydroxide in water in the electrolytic flotationcell. The sludge in compartment 144 consists of fine suspended solidsalong with ferric hydroxide or aluminum hydroxide. When the sludge isdewatered in filter press 168, the filter cake can be flash dried athigh temperatures to destroy bacteria and then pelletized. The finalpelletized materials, having high concentrations of either iron oraluminum can be used as ores for steel or aluminum production, oralternatively can be used in the manufacture of various useful chemicalproducts, including coagulants for use in this system.

There are, of course, many possible inorganic coagulants in addition toferric chloride and aluminum sulfate. However, ferric chloride andaluminum sulfate are considered to be the most practical coagulants atthe present time, ferric chloride being preferred.

Sludge discharged through ports 60 and 64 at the bottom of separationvessel 44, and through port 150 at the bottom of compartment 134 offlotation cell 106 is used, along with the solid material discharged bybar screen 10 in sludge path 12, to manufacture activated carbon for usein the system. Sludge discharged through port 60 is delivered throughpump 182 to path 184. Sludge discharged through port 64 is delivered bypump 186 to path 184. Pumps 182 and 186 are also arranged to returnsludge through path 190 to inlet 42 of separation vessel 44 if desired,suitable valving (not shown) being provided. Accumulated oil and grease,which pass over weir 70 in vessel 44 along with other floating solids,are delivered through path 72 and pump 192 to separation vessel inlet42, but by opening valve 194, these materials can alternatively bedelivered to path 184.

Path 184 is the inlet of a self-cleaning rotary brush microstrainer 196,which is preferably a fine screen device producing an output ofrelatively dry solids, as compared with centrifugal screen concentrator44, but having a somewhat lesser capability in terms of liquid volume.The "MICRODREX" microstrainer, manufactured by Idrex, Inc. of 1018Lambrecht Drive, Frankfort, Illinois is an example of a suitablemicrostrainer. Microstrainer 196 comprises a housing 198, in which thereis located an array of triangular screen elements 200, in the form of aportion of a circular cylinder. The triangular screen elements arepreferably spaced from each other by approximately 100 microns or less.Liquid containing finely divided solids entering the microstrainer frompath 184 through inlet 202 is guided upwardly through passage 204 ontothe upper side of screen 200, and falls through the screen into chamber206, and outwardly through outlet 208. Chamber 206 is defined by walls210 and 212 at the respective ends of the screen, so that liquid isrequired to fall through the screen in order to enter chamber 206. Acylinder 214, carrying a series of brushes 216 on its outer surface isarranged to rotate in a clockwise direction, and is positioned so thatits brushes come into contact with screen 200, and remove solidscontinuously by brushing them off the left end of the screen (at wall212), and into downward passage 218, which leads to an inclined,motor-driven screw conveyor 220. Conveyor 220 leads to a vertical screwconveyor 222, having an outlet 224 in communication with path 226.

Liquid from microstrainer 196 is delivered through outlet 208 and path228 back through path 104 to the inlet of flotation cell 106. Water isdrained from the solid material in inclined conveyor 220 through outlet230, and the solids are delivered through path 226 to a molten saltactive carbonizer 232. Carbonizer 232 receives carbonaceous sludge frommicrostrainer 196, from bar screen 10 and from rotary strainer 22(through path 32), and converts the sludge into a high quality activatedcarbon by first flash drying the carbon in a sludge conditioner, andpassing the product of the conditioner into a molten salt column,wherein the sludge particles are carbonized and caused to float to thesurface of the column for removal into path 36.

The details of carbonizer 232 are shown in FIG. 2. The principalcomponents of the system are a sludge conditioner 234, a cycloneseparator 236, a pneumatic conveyor 238, a molten salt column 240, and amolten salt scrubber 242.

Sludge conditioner 234 is preferably a closed-loop flash dryer of thetype described in the copending application of Francis E. Albus andGeorge W. Fendler, Ser. No. 911,889, filed June 2, 1978. The dryercomprises a generally toroidal conduit 244 having a horizontallydisposed air inlet nozzle 246 arranged to direct a stream of heated airinto conduit 244 in a tangential direction in order to induce arecirculating flow in the loop. A material feeder 248 is arranged tofeed wet sludge into the circulating stream of hot air in conduit 244.In the preferred form of dryer, the feeding device is arranged so thatthe sludge is fed into the loop at a point preceding the hot air inletnozzle so that the sludge entering the loop is immediately subjected tothe influence of the hot air entering the loop through the nozzle. Anoutlet 250 is provided in communication with the interior of conduit244, the outlet opening being on the interior side of the loop, andpreceded in the direction of flow by a curved portion 252 of the conduitso that centrifugal force tends to maintain large particles and wetparticles toward the exterior of the loop, while allowing small, dryparticles to exit from the loop through outlet 250. Outlet 250 is incommunication with an outlet conduit 254.

Air is delivered to the dryer inlet nozzle 246 through path 318 by a lowpressure motor driven blower 256, which is followed by a heat exchanger257. Heat exchanger 257 receives the exhaust of scrubber 242, andtransfers a part of the heat to air in path 318. An auxiliary air heateris provided at 258. This auxiliary air heater can be fueled by naturalgas, propane, oil, coal, or other combustible material. The temperaturesof the air at inlet nozzle 246 can be as high as 815° C., or possiblyeven higher.

While various alternative forms of sludge conditioners can be used inthe system, the loop dryer is preferred because of its inherent abilityto classify particles, i.e. to separate wet particles and largeparticles from fine, dry particles. The loop dryer thereby provides aproduct at conduit 254 which is relatively uniform in particle size andin moisture content. Alternative forms of flash dryers, i.e. dryerswhich operate by subjecting the wet particles to the effect of a movingstream of hot air or other gas, have in common with the loop dryer thevirtue of efficient drying because of the combined effects of heat andair transpiration. The loop dryer, however, generally produces a moreuniform product.

The fine, dry particles which exit from the dryer through conduit 254are delivered to inlet 260 of cyclone separator 236, in which theparticles are separated from the air or other gas in a well-knownmanner. The particles are collected in a dust trap 262, from which theycan be removed by the operation of rotary valve 264. The air or othergas which accompanied the particles in conduit 254 is delivered from thecyclone separator at the top through outlet conduit 266.

Various alternative devices for separating gas from particulate matter,such as the Aerodyne Dust Collector, available from Aerodyne DevelopmentCorporation of Cleveland, Ohio can be used in place of cyclone separator236.

Pneumatic conveyor 238 comprises a hopper 268 arranged to receive drysludge particles passed from dust trap 262 through rotary valve 264 andopening 270 in hopper cover plate 272. The interior of the hoppercommunicates with a pressurizing chamber 274 through an opening 276 inbaffle 278, the opening being closable by a material valve 280,controlled through rod 282 by a hydraulically operated valve actuatingcylinder 284. A compressed air inlet 286 is provided in pressurizingchamber 274, to force material out of chamber 274 into passage 288.

In operation of the pneumatic conveyor, valve 280 is openedintermittently to allow sludge particles to fall from the hopper intopressurizing chamber 274. After the valve is closed, air or inert gas isintroduced through air opening 286 to force the material downwardlythrough passage 288 into molten salt column 240, in which thecarbonization of the sludge takes place.

Molten salt column 240 comprises a generally cylindrical verticallydisposed column 290 having an opening 292 at an intermediate level,through which material is introduced into the molten salt bath by thepneumatic conveyor through passage 288. Molten salt is maintained to alevel indicated at 294, well above opening 292. The salt bath ismaintained in the molten condition by a suitable heating means, forexample by an electrical resistance heater 296. A series of stirringblades 298 are submerged in the molten salt bath and operated throughshaft 300 by motor 302. Level 294 of the salt bath is situated justbelow outlet opening 304, to which is connected a motor-driven screwconveyor assembly 306 for the delivery of a solid product through outlet308. A gas exhaust passage is provided at the upper end of the column at310, and a high temperature valve 312 is provided at the lower end ofthe column for the intermittent removal of heavy-metal containingmaterial from the lower end of the molten salt column.

Exhaust passage 310 of the molten salt column communicates throughcontrol valve 311 with inlet 314 of scrubber 242, which is preferably ofthe type described in U.S. Pat. No. 3,647,358 to Jacob Greenberg, datedMar. 7, 1972. This scrubber also receives the mixture of gas andparticles exhausted from cyclone separator 236 through its outlet 266.Outlet conduit 316 of the scrubber is in communication through path 318with the high temperature gas inlet of heat exchanger 257, a fan 320being provided in the conduit. Heat from the exhaust gases of thescrubber is transferred to the air entering the sludge conditionerthrough path 318. Consequently, exhaust heat from the scrubber isutilized in the loop dryer to reduce the energy requirements of thesystem.

The salts and salt mixtures described in Greenberg U.S. Pat. No.3,647,358 can be used in the carbonizer of FIG. 2, but preferablyhydrated salts such as sodium hydroxide, potassium hydroxide and lithiumhydroxide or mixtures thereof are used in a temperature range betweenabout 205° C. and 650° C. Oxidizers such as potassium chromate, sodiumnitrate and the like can be used, but large proportions of oxidizers inthe salt mixture will interfere with the carbonization process andreduce the yield of activated carbon at product outlet 308.

In operation, preconditioned sludge is introduced into the carbonizationcolumn intermittently by the operation of pneumatic conveyor 238, and isdelivered to an intermediate level in the molten salt column.Carbonization of the sludge particles takes place in the column, andactivated carbon particles are formed. These activated carbon particlesfloat to the surface 294 of the salt bath, forming a layer which isremoved by the operation of screw conveyor 306. In the column, ash,including heavy metal salts such as cadmium, chromium and lead salts,sinks to the bottom of the column. Those heavy metal salts which do notdissolve sink to the bottom of the column directly. Some heavy metalsalts dissolve in the salt bath, however, and sink to the bottom onlywhen the concentration of heavy metal salts in the column reaches asaturation point. The ash is removed intermittently by opening hightemperature valve 312. Separation takes place within the column bygravity, and a relatively pure activated carbon is delivered at productoutlet 308.

Importantly, the preconditioning of the sludge by flash drying resultsin a sludge having a very large surface area per unit weight because ofthe rapid expansion of the sludge particles as they encounter the movingstream of hot air in the dryer. The large surface area of the sludgeparticles insures complete carbonization in the column, and alsoimproves the adsorbent qualities of the activated carbon which isproduced. A loop dryer, as opposed to conventional flash dryers, ispreferred in the system of FIG. 2 by reason of its ability to deliverparticles having a relatively uniform size range and moisture content.

The activated carbon delivered through product outlet 308 is preferablywashed with water, steam, or acid in the case where hydrated salts areused in the column, and is dewatered by screening or other means anddried. The activated carbon can be dried in a loop dryer for example.

As will be apparent, the system of FIG. 2 utilizes the sludge producedby the operation of various components of the system of FIG. 1 toproduce a high quality activated carbon, which is used, in turn, as anadsorbent for the removal of organic components of the wastewater beingtreated by the system. It will also be noted that, as the system is aclosed loop, the sludge delivered to the loop dryer in FIG. 2 contains aproportion of spent activated carbon particles. The spent activatedcarbon is delivered to the molten salt column, and is there reactivatedfor further use. Adsorbed organic materials are removed by scrubber 242,and any heavy metal salts which would otherwise become accumulated inthe sludge or in the activated carbon are collected at the bottom of themolten salt column, and removed through valve 312.

The principal advantages of the invention can be summarized as follows.First, the introduction of flocculating agents following screening byfine mesh screening means produces a highly effective removal of organicmatter from the wastewater with only modest space requirements, and theuse of a centrifugal screen separator for fine mesh screening producesresults far superior to those achieved heretofore.

The effectiveness of the centrifugal screen separator is markedlyimproved by preceding it in the general direction of liquid flow by acorrugated plate interceptor, which removes greases and oils from thewastewater, which would otherwise interfere with the operation of thecentrifugal screen device.

In the preferred system, fine particle screening is carried out by theuse of two separate screening devices, one being preferably a verticalcentrifugal screen strainer, and the other being a mechanicallyself-cleaned device having triangular screen elements. Prior to itsintroduction to these screening devices, the wastewater is subjected toa thickening step, preferably carried out at least in part by the use ofa corrugated plate interceptor. The thickened material is delivered tothe self-cleaning device, while the remaining wastewater is delivered tothe centrifugal screen device. The effluents of the screening devicesare then recombined for further treatment by flocculation.Significantly, the corrugated plate device serves a dual function inthat it separates grease and oil from the wastewater, thereby improvingthe operation of the centrifugal screen device, and at the same timeeffects thickening of the solids content of the wastewater for the mosteffective operation of the low liquid volume mechanically self-cleanedmicrostrainer. In this way maximum recovery of water is achievedefficiently. The use of the thickener also allows sufficient contacttime between the activated carbon and the wastewater being treated topermit effective adsorption to take place.

In the preferred form of the system, organic sludge, which is removed bythe various screening and settling devices in the system is recycledthrough a molten salt active carbonizer to produce a high-qualityactivated carbon, which is used to adsorb organic substances in thewastewater. The molten salt active carbonizer, by removing heavy metalsfrom the recirculating system, makes it possible to recycle sludgeeffectively.

Flocculating agents are added to the effluent of the screening devices,and removal of chemical sludge takes place in an electrolytic flotationcell incorporating a corrugated plate interceptor, in whichelectrolytically produced bubbles cause a reverse flow of solid materialto take place for effective final treatment of the wastewater.

Finally, contact time of the activated carbon in the system ismaintained by the recycling of carbon from the centrifugal screen deviceto separation vessel 44, and effective control of the activated carbonretention is achieved by the capability of removing spent activatedcarbon from the separation vessel.

I claim:
 1. A wastewater treatment process comprising the stepsof:separating coarse solids from wastewater to produce a first productsubstantially free of coarse solids; effecting thickening of theremaining fine solids to produce a second product consisting essentiallyof fine solids and water, and a third separate product consistingessentially of water with a proportion of fine solids substantially lessthan the proportion of fine solids in the second product; passing thesecond product through a first fine particle screen to separate solidsfrom water in said second product while continuously cleaning said firstscreen by mechanical means; passing the third product through a secondfine particle screen, thereby separating fine solids from water in saidthird product; and recovering, as a final product, a liquid comprisingsubstantially all of the water produced in both of said passing steps.2. A process according to claim 1 in which the step of passing saidthird product through said second fine particle screen is carried out bymeans of a centrifugal screen separator.
 3. A process according to claim1 in which said thickening step is carried out at least in part by meansof a corrugated plate interceptor.
 4. A process according to claim 1 inwhich the solids separated from water in said third product arerecombined with said first product and subjected to thickening in saidthickening step.
 5. A process according to claim 1 in which the step ofpassing said third product through said second fine particle screen iscarried out by means of a centrifugal screen separator, in which saidthickening step is carried out at least in part by means of a corrugatedplate interceptor, and in which said corrugated plate interceptor alsoserves to eliminate oil and grease from said third product, therebypreventing substantial quantities of oil and grease from reaching andinterfering with the operation of said centrifugal screen separator.